(1) Field of the Invention
The present invention relates to an inter-LAN connection apparatus called a MAC (medium access control) bridge which transfers frames among local area networks (LANs), and to an inter-LAN connection method. More particularly, the invention relates to a filtering control system in the MAC bridge.
A LAN (local area network) is a communication network for connecting computers, terminals, and peripheral equipment that are installed in relatively limited places such as institutes and factories. Depending upon the topology, the LANs can be classified into those of the bus type such as token buses, and those of the ring type such as token rings. The medium access method includes a carrier sense multiple access with collision detection (CSMA/CD) and token passing.
When the above various types of LANs are connected, frames are to be transferred among the LANs. A plurality of LANs are often connected to one LAN.
In the inter-LAN connection apparatus, when the destination of a received frame is a station in the LAN which is a source of transmission, it is desired to abort the frame. If it is not aborted, the load on other LANs that have no relation to that communication, will be increased.
(2) Description of Related Art
In a conventional art, when a plurality of LANs are to be connected to one LAN, a single inter-LAN connection apparatus is connected between the above one LAN and the plurality of LANs.
In the above-mentioned conventional system, however, a microprocessor in the inter-LAN connection apparatus retrieves address tables for all of the frames sent from the LANs what confirm to what LAN to which the destination station is connected, and judges the frames to be either aborted or transferred. Therefore, as the number of LANs that are connected increases, or the number of stations increases in the LANs, or as the communication speed increases, the load increases for the retrieval of the destination and for judging that the frames be aborted or transferred based upon the result of the retrieval, requiring to the microprocessor to have an increased processing ability in the inter-LAN connection apparatus.
In recent years, in particular, speed has tended to be increased along with the development of optical communication technology or encoding technology, and merely increasing the processing speed of a microprocessor is no longer adequate to cope with these problems, but simply causes a drastic increase in the cost of the apparatus. It has therefore been desired to provide an inter-LAN connection apparatus that is capable of easily coping with an increase in the speed of communication and the expansion of a network.
Described below in detail are the problems in a conventional inter-LAN connection apparatus. As described above, the inter-LAN connection apparatus (MAC bridge) works to transfer frames among a plurality of LANs. The communication frames in the same LAN, however, must be blocked (filtered) from being transferred and must be aborted. In general, the transferring or aborting operation is processed by referring to the station addresses registered in a filtering table and to the port numbers of the LAN to which the station is connected.
The relay function for the transfer or abortion has heretofore been realized by using a common unit of the inter-LAN connection apparatus (MAC bridge). That is, all the frames flowing from the LANs are received by a common buffer in the inter-LAN connection apparatus and are examined whether to be transferred or aborted. This processing ability is usually called a filtering rate. The LANs in recent years have been designed to operate at ever faster speeds, and it has been demanded to increase the filtering rate.
When frames are relayed by the inter-LAN connection apparatus (MAC bridge), in general, the transmitting speed is not always greater than the receiving speed since there are differences or variations in the speed and load of a LAN. In general, it often happens that a number of frames greater than the transmitting speed must be relayed, and a buffer having a sufficient capacity must be provided. According to the prior art in which the frames of all LANs connected to the inter-LAN connection apparatus are all received by the buffer and filtered, however, an additional processing must be carried out to effectively utilize the regions of the aborted frames, causing the filter processing and the relay processing ability to be decreased. Furthermore, the firmware processing imposes a limitation on the filter processing and on the relay processing ability.